Microdisplacement and Adaptive Subdivision is not working
No need for a bump node there, I would rather substitute it with a "math -> multiply" node, with which you can change the displacement height.
In fact, what you are doing is taking a greyscale image, making a normal map out of it and plugging it into the displacement slot, which needs a greyscale image. But now you have wrong shades of grey because of that.

You also need correct render settings:
In the render tab, in the render pane, you need to change the feature set to "experimental" and in the material tab, in the settings pane, the displacement to "both". (And obviously you need your mesh to be unwrapped properly.)